Coherent Raman imaging with fibers: From sources to endoscopes

Coherent Raman imaging with fibers: From sources to endoscopes Esben Ravn Andresen Institut Fresnel, CNRS, École Centrale, Aix-Marseille University, France Journées d'imagerie vibrationelle 1 Jul, 2014 www.fresnel.fr/mosaic

1. Outline Concepts Sources Adapting an existing laser for CRS using fibers Fiber lasers Making a compact, cheap, specialized source for CRS using fiber lasers Endoscopes Integrating fiber delivery and fiber collection

2a. Concepts / Classes of fiber Standard fiber(smf) PCF Endlessly single-mode; Controllable dispersion MMF SMF, single-mode fiber; HC, hollow-core; PBG, Photonic-bandgap; SC, solid-core; MCF, multi-core fiber; PCF, photonic-crystal fiber; MMF, multi-mode fiber HC-PBG SC-PBG Guides in air; Large anomalous Narrow transmission dispersion window; Large anomalous dispersion Imaging fiber/bundle Kagomé Guides in air; Broad transmission window Small dispersion; MCF

2b. Concepts / Modes Mode dispersion: Different modes have different propagation constants (different propagation velocities) Mode profile: Only fundamental mode can be effectively focused to a small spot. => SMFs preferred for delivery of pulses in CRS Mode filtering: Coupling efficiency to fiber is determined by overlap between incoming light field and fiber modes => MMFs preferred for collection of diffuse CRS signal [Agrawal]

2c. Concepts / Dispersion Group-velocity dispersion GVD, β2 β2= d2/dω2(β) = d2/dω2[2πn(ω) / λ] β2 > 0 ; D < 0 Dispersion parameter D D = -2πc/λ2β2 β2 < 0 ; D > 0 Normal dispersion; anomalous dispersion Dispersion length Ld Ld = τ2 / β2 Material dispersion Waveguide dispersion: Additional contribution in PCF and PBG [Agrawal]

2d. Concepts / Nonlinearity Nonlinear refractive index n2 n = n0 + n2i ; n2 = 3*10-16 cm2/w Nonlinear coefficient γ γ = 2πn2 / (λaeff) Nonlinear length Lnl = 1 / (γp0) Huge interaction length in fibers Self-phase modulation; Four-wave mixing; Soliton dynamics; Dispersive wave generation Precise modeling of nonlinear propagation in fiber possible but requires numerical methods [Dudley, Rev. Mod. Phys. (2006)] [Agrawal]

3a. Sources / Dispersive wave First demonstration of a fiber-source for CRS Dispersive wave: Not TFL, wavelength-tunable Application example: fs-cars; Pump: Dispersive wave; Stokes: fs-laser Disp. wave fs-laser [Paulsen, Opt. Lett. (2003)]

3b. Sources / Spectral compression Low-loss alternative to spectral filtering Almost TFL Wavelength fixed by source Do not confuse 'spectral compression' with 'spectral focusing'. Application example: Convert a fs-pump pulse into a ps-pump pulse for hyperspectral CARS [Stolen, Phys. Rev. A (1978); Andresen, Opt. Lett. (2005); Andresen, J. Opt. Soc. Am. B (2005)]

3b. Sources / Spectral compression Alternative spectral compression: 'Adiabatic soliton spectral compression' Requires dispersion-increasing fiber (DIF) with anomalous dispersion [Chuang, Opt. Lett. (2011)]

3c. Sources / Soliton light source Source of red-shifted fs-pulses Requires fiber with anomalous dispersion Inherently transform-limited Wavelength-tunable over several 100 nm Application example: CARS, SRS; Pump: fs-laser; Stokes: Soliton [Andresen, Opt. Lett. (2006); Andresen, Opt. Lett. (2011); Ouzounov, Science (2003)]

3c. Sources / Soliton light source Source of red-shifted fs-pulses Requires fiber with anomalous dispersion Inherently transform-limited Wavelength-tunable over several 100 nm Application example: CARS, SRS; Pump: fs-laser; Stokes: Soliton [Andresen, Opt. Lett. (2006); Andresen, Opt. Lett. (2011); Ouzounov, Science (2003)]

3c. Sources / Soliton light source Source of red-shifted fs-pulses Requires fiber with anomalous dispersion Inherently transform-limited Wavelength-tunable over several 100 nm Application example: CARS, SRS; Pump: fs-laser; Stokes: Soliton [Andresen, Opt. Lett. (2006); Andresen, Opt. Lett. (2011); Ouzounov, Science (2003)]

3c. Sources / Soliton light source Source of red-shifted fs-pulses Requires fiber with anomalous dispersion Inherently transform-limited Wavelength-tunable over several 100 nm Application example: CARS, SRS; Pump: fs-laser; Stokes: Soliton [Andresen, Opt. Lett. (2006); Andresen, Opt. Lett. (2011); Ouzounov, Science (2003)]

3c. Sources / Soliton light source Source of red-shifted fs-pulses Requires fiber with anomalous dispersion Inherently transform-limited Wavelength-tunable over several 100 nm Application example: CARS, SRS; Pump: fs-laser; Stokes: Soliton [Andresen, Opt. Lett. (2006); Andresen, Opt. Lett. (2011); Ouzounov, Science (2003)]

3c. Sources / Soliton light source Solitons are limited in energy by the relation 1 = τ E (γ / β2 ) The fiber parameter γ / β2 is the limiting factor in PCF Other types of fiber with lower γ / β2 : HC-PBG γ / β2 is too low, requires µj-level laser (Ti:S ~ 20 nj) LMA-fiber only works at λ > 1300 nm SC-PBG 5x gain in energy around 800 nm HC-PBG LMA-fiber SC-PBG [Andresen, Opt. Lett. (2006); Andresen, Opt. Lett. (2011); Ouzounov, Science (2003)]

3d. Sources / Supercontinuum Source covering entire spectral region of interest Highly non-transform limited; if pulse-to-pulse coherence is high, can be compressed to a short pulse Attention, pulse-to-pulse coherence depends on pump pulse! Longer pump lower coherence Pump in anomalous dispersion region lower coherence Attention, timing of continuum sub-pulses! Application example: Hyperspectral CARS Pump: ps-laser; Stokes: Filtered continuum [Kee, Opt. Lett. (2004)]

3e. Sources / Spectral focusing High spectral resolution CRS with fs-pulses Well-suited to e.g. soliton light source Effectively similar to CRS with transform-limited ps-pulses Tunability over pulse bandwidth (~300 cm-1) Do not confuse 'spectral focusing' with 'spectral compression'! Application example: (Hyperspectral) CARS, SRS [Andresen, Opt. Lett. (2011)]

4a. Fiber lasers Interest of fiber lasers: Potentially small, cheap, environmentally stable, and alignment-free Two main types of fiber lasers: Yb: λ ~ 1050 nm +- 10 nm Er: λ ~ 1550 nm +- 10 nm On their own, these two types would only span 300 cm-1 in CRS Wavelength conversion schemes required to span the wavelength range required for CRS (3000 cm-1)

4b. Fiber lasers First all-fiber source for CRS Yb fiber laser; wavelength conversion by soliton Provides fixed ps-pump and tunable fs-stokes (hyperspectral CARS) [Andresen, Opt. Express (2007)]

4c. Fiber lasers Fiber laser based source tunable over 0 3000 cm-1 Er fiber laser; wavelength conversion by SHG in PPLN (pump), continuum generation + SHG (Stokes) Provides fixed ps-pump and tunable ps-stokes (ps-cars) [Krauss, Opt. Lett. (2009)]

4d. Fiber lasers Alignment-free all-fiber laser source Yb fiber laser; wavelength conversion by optical parametric generation (OPG) Provides fixed ps-pump and fixed ps-stokes (ps-cars) [Baumgartl, Opt. Express (2012)]

4e. Fiber lasers Another alignment-free all-fiber laser source Yb fiber laser; wavelength conversion by optical parametric amplification (OPA) Provides tunable (over 300 cm-1) ps-pump and fixed ps-stokes (ps-cars) Performance in 2650-2950 cm-1 range similar to bulk OPOs [Lefrancois, Opt. Lett. (2012)]

4f. Fiber lasers Optical parametric oscillation-based Yb fiber laser; wavelength conversion by OPO Provides tunable ps-pump and tunable ps-stokes (ps-cars) Performance in 2740-3150 cm-1 range similar to bulk OPOs [Lamb, Opt. Lett. (2013)]

4g. Fiber lasers Fast spectral filtering Yb fiber laser; wavelength filtering; locked to bulk Ti:sapph Provides fixed ps-pump and tunable (300 cm-1) ps-stokes (ps-cars/srs) Hyperspectral imaging Performance similar to bulk OPOs [Ozeki, Nat. Photon. (2012)] Noise characteristics ok for SRS

4h. Fiber lasers Based on both Yb and Er Er fiber laser; Yb and Er fiber amplifiers; continuum generation; SHG Provides fixed ps-pump and tunable ps-stokes (ps-cars/srs) Performance in 2800-3100 cm-1 range similar to bulk OPOs Noise characteristics ok for SRS (auto-balanced detection) [Freudiger, Nat. Photon. (2014)]

4i. Fiber lasers / Time lens Time lens source gives 2nd ps-pulse; inherently synchronized to Ti:sapph Noise characteristics ok for SRS [Wang, Opt. Express (2010); Wang, J. Biophotonics (2013)]

5a. Endoscopes/Fiber scanning Concept: Image fiber tip onto sample, scan fiber Collection of signal by tip-mounted photodiode (PD) limits miniaturization CARS and SRS imaging demonstrated Back-collection of diffuse signal through the delivery fiber would be very inefficient (mode filtering) [Saar, Opt. Lett. (2011)]

5a. Endoscopes / Fiber scanning Double-clad HC-PBG as endoscope fiber Inner clad is multi-mode and effectively collects back-scattered diffuse signal CARS and SRS imaging demonstrated Further potential (broader spectral transmission window) with the Kagomé-type fiber [Brustlein, Opt. Express (2011)]

5b. Endoscopes / Confocal Based on an imaging fiber bundle Basis for a commercial endoscope (confocal, 1-photon) So far, CRS imaging not demonstrated although in principle feasible [Göbel, Opt. Lett. (2004); cellvizio.net]

5c. Endoscopes / Lensless Based on wave front control; no elements on fiber tip So far, CRS imaging not demonstrated although in principle feasible: Distribution of excitation over N cores less nonlinear distortion Pump and Stokes in different subsets of cores less interaction during delivery Wave front control compensation for chromatism; optical sectioning [Thompson, Opt. Lett. (2011); Andresen, Opt. Lett. (2013); Andresen, Opt. Express (2013)]

6. Conclusions Sources Many solutions exist that can enhance certain aspects of an existing setup at a reasonable time and resource expenditure Future prospect: High-power soliton sources Fiber lasers Are getting competitive in terms of performance in the 2700-3100 cm-1 region Many competing concepts - will they converge on one optimal solution? Endoscopes There will likely be developments in all kinds of endoscopes (fiberscanning, confocal, lensless) towards CRS

7. Bibliography Concepts: Endoscopes: Agrawal, 'Nonlinear fiber optics' (Academic, San Diego) Dudley et al., Rev. Mod. Phys. 78, 1135 (2006) Saar et al., Opt. Lett. 36, 2396 (2011) Brustlein et al., Opt. Express 19, 12562 (2011) Göbel et al., Opt. Lett. 29, 2521 (2004) cellvizio.net Thompson et al., Opt. Lett. 36, 1707 (2011) Andresen et al., Opt. Lett. 38, 609 (2013) Andresen er al., Opt. Express 21, 20713 (2013) Sources: Paulsen, Opt. Lett. 28, 1123 (2003) Stolen et al., Phys. Rev. A 17, 1448 (1978) Andresen et al., Opt. Lett. 30, 2025 (2005) Andresen, J. Opt. Soc. Am. B 22, 1934 (2005) Chuang et al., Opt. Lett. 36, 2848 (2011) Andresen, Opt. Lett. 31, 1328 (2006) Andresen et al., Opt. Lett. 36, 2387 (2011) Ouzounov et al., Science 301, 1702 (2003) Kee et al., Opt. Lett. 29, 2701 (2004) Fiber lasers: Andresen et al., Opt. Express 15, 4848 (2007) Krauss, Opt. Lett. 34, 2847 (2009) Baumgartl et al., Opt. Express 20, 21010 (2012) Lefrancois et al., Opt. Lett. 37, 1652 (2012) Lamb et al., Opt. Lett. 38, 4154 (2013) Ozeki et al., Nat. Photon. 6, 845 (2012) Freudiger et al., Nat. Photon. 8, 153 (2014) Wang et al., Opt. Express 18, 24019 (2010) Wang et al., J. Biophotonics 6, 815 (2013) Reviews: Xu et al., Nat. Photon. 7, 875 (2013).

Soliton light source Spectral focusing Sarah Saint-Jalm Lensless endoscope Spectral compression Esben Ravn Andresen Fiber-scanning endoscope Alberto Lombardini & Xueqin Chen 7. People